1. Field of the Invention
The present invention relates to an auto-focusing device and, more particularly, to an auto-focusing device having a light emitting element for emitting an optical beam to be projected upon an object.
2. Description of the Prior Art
There have been attempts to provide an auto-focusing device using an optical beam. For example, a method by which an optical beam is projected upon an object and the reflected beam is detected by an image position detecting element for outputting image position signals, is disclosed in Japanese Patent Laid-Open No. 61-221717 entitled "Auto-Focusing Device for Cameras". The image position detecting element is a semiconductor element called a "PSD" (Position Sensitive Device). In this image position detecting element, first and second junctions are formed at the righthand and lefthand sides facing the object to generate image position signals (in photocurrent). These image position signals are dependent upon how much of the optical signal reflected from the object is incident upon the light receiving face of the image position detecting element. The image position signals are amplified, detected and integrated and are output to an image position signal processor for outputting a range of data (in voltage). A range computing circuit computes a range signal for determining the lens position from the range of data.
In general, one method of deciding whether or not a valid range signal can be generated requires first establishing a limit level. Then, the first and second computed values are evaluated to determine whether the range signal is within the established limit. A second method of deciding whether or not a valid range signal can be generated requires estimating the range from the absolute value of the level (or intensity) of an incident optical signal. Then the first and second computed values are evaluated to determine whether the range signal is within the estimated range.
In either method, it is decided according to the S/N ratio of the incident beam and the threshold level whether or not the range signal is valid. If it is judged that the computed range signal is not valid, the object is assumed to be at the remotest position.
To improve the reliability of the range signal, the threshold level of the acceptable S/N ratio is set at a particular value. However, this creates a problem in positioning the lens for maximum focusing accuracy when the object is at a distance. With the threshold level set to improve the reliability of the range signal, the S/N ratio detected from distant objects near the range of the remotest position is such that focusing accuracy is deteriorated.